In recent years, spread spectrum communication systems are more and more important in personal mobile communication networks. In particular, direct sequence code division multiple access (DS/CDMA) has been adopted in third generation cellular standard. Rake receiver is frequently used in DS/CDMA system to overcome multipath problems.
Rake receiver contains a plurality of rake finger tracking units. Each rake finger tracking unit demodulates one multipath signal. The multipath information from each finger is then combined together using the maximum ratio combining technique to obtain a reconstructed signal with the maximum signal to noise ratio and, consequently, the smallest bit error rate.
FIG. 1 is a schematic diagram of a rake receiver according to the prior art. The rake receiver 10 comprises a searcher 14, a plural of rake finger tracking units (RFTU) 12, a rake finger management (RFM) 16 and a maximum ratio combiner (MRC) 18, wherein the maximum ratio combiner 18 is commonly used, but there are other kinds of maximum ratio combiners could also be used in the present invention.
When the rake receiver 10 receives an external multipath signals 13 from an antenna 11, the searcher 14 searches a plurality of peaks 15, 17, 19 in of the external multipath signal 13 with a first predetermined resolution, and the time positions of the peaks are figured out. The objective of the searcher 14 is to identify the existence of the path signals 15, 17, 19, so it is not necessary to use a high resolution. If the first predetermined resolution were too high, it would make the rake receiver 10 huge for calculating the signal and higher power consumption needed. The searcher 14 and the rake finger tracking units 12 need to match up with each other for the purpose of making rake receiver smaller and simpler, lower power consumption and accurate acquiring the time positions of peaks 15, 17, 19.
The rake finger management 16 assigns corresponding peaks of the multipath path signal for the rake finger tracking units to obtain the accurate time positions and the power of the corresponding multipath signals, and each tracking unit only tracks one peaks of the multipath signal. Therefore, the rake finger tracking units 12a, 12b, 12c each has a predetermined tracking time position, and each of the rake finger tracking units, 12a, 12b, 12c, tracks a corresponding peak with a second predetermined resolution wherein the second predetermined resolution is higher than the first predetermined resolution. The rake finger tracking units 12a, 12b, 12c would acquire the time positions more precisely compared with the path signal acquired from the searching time positions.
The rake finger tracking units would finally combine the corresponding path signals 15, 17, 19, by the maximum ratio combiner 18 to obtain the corresponding original signals. The original signals would have the maximum signal-interference ratio(SIR) after the aforementioned process.
Due to the variation of the multipath signals, the predetermined tracking time position might not catch the correct path signals 15, 17, 19, or the path signals are far from the predetermined tracking time position and are out of the effective tracking range. It means that there might be some idle rake finger tracking units or some rake finger tracking units cannot identify the path to track which causes the waste and idle units in the rake receiver.
Besides the three major components, the rake finger management 16 plays an important role in the rake receiver. The subject of the rake finger management 16 is to assign and manage the rake finger tracking units 12. However, the rake finger management unit according to the prior art could not solve the problem effectively. The rake finger management unit according to the prior art unnecessary reassigns the rake finger tracking units and leads to the longer tracking time and lower efficiency. Consequently, this invention provides a method to decrease unnecessary reassignment and improve the system efficiency if there is any idle rake finger tracking units or the path signals are far from the predetermined tracking time position.